BRPI0903362A2 - process for hydrotreating a hydrocarbon fuel - Google Patents

Abstract

PROCESS FOR THE HYDRO-TREATMENT OF A FUEL hydrotreatment. The hydrocarbon fuel introduced in the hydrotreatment stage is a fuel containing renewable organic material that generates carbon monoxide during the process operation. The carbon monoxide is then converted to hydrogen in the recycling circuit by a water-gas displacement stage.

Description

"PROCESS FOR HYDRATING A FUEL HYDROCARBON"

The present invention relates to a process for hydrotreatment of hydrogen co-produced fuels during process operation. More particularly, the invention relates to a hydrotreating process wherein the hydrocarbon fuel contains renewable organic material that generates carbon monoxide during process operation. Carbon monoxide is then converted to recycle hydrogen by a water-gas displacement stage.

During refining of oil feeds, the hydrotreating stages are used for the removal of impurities such as sulfur and nitrogen. Hydrocarbon feedloads, and particularly heavy hydrocarbons such as petroleum and diesel, often contain organic sulfur and nitrogen compounds which, in subsequent stages, represent unwanted impurities due to their negative effect on catalyst activity. In addition, environmental guidelines impose a demand on the production of extremely clean transport fuels with very low sulfur levels, for example as low as 10ppm total sulfur in diesel fuels. Sulfur and nitrogen impurities are thus hydrogenated during hydrotreating in hydrogen sulfide and ammonia before being treated at a subsequent hydroprocessing stage, where, depending on the hydrocarbon feedstock used, their value may be increased by rearrangement of the molecules, for example by hydrocracking.

Hydrogen required in the hydrotreating process is usually supplied internally in the process by mixing the hydrocarbon fuel with hydrogen-rich recycle gas produced in the process. Additional hydrogen can be fed from an external source in the form of a replacement gas, which can be combined with the gas. of high hydrogen recycle produced in the hydrotreating process. Melting hydrogen is used to compensate for the hydrogen consumed during hydrotreating. After passing the common-contact hydrotreating stage or more fixed beds of, for example, hydrodesulfurization (HDS) or hydrodesionitrogen (HDN) catalysts, the hydrocarbon feedstock, for example an oil or diesel feed being depleted of sulfur and nitrogen, is conducted to a hot separator from which a top fraction and a bottom fraction are removed.

The top fraction is then cooled and mixed with water entering the cold separator, where a top fraction, in the form of a hydrogen-rich gaseous stream, is removed. This hydrogen rich stream is then recycled to the hydrotreating stage, while the bottom fraction containing the clean fuel is removed and ready for use, for example as a transport fuel.

US-A-2002/004533 describes a process for the integration of displacement reactors and hydrotreaters, wherein the hydrogen-rich recycle stream from the hydrotreatment process is combined with hydrogen produced from the water-gas displacement of a synthesis gas produced in a separate process.

US 3,694,344 discloses a process in which part of the hydrotreated hydrogenate in the hydrotreating stage is obtained from a separate process for hydrogen production comprising reforming natural gas to produce synthesis gas with subsequent water-gas displacement to another conversion into a rich gas. in hydrogen.

US 3,413,214 describes a process for the hydrogenation of liquid hydrocarbons in which oxygen gas is added to the liquid hydrocarbon to induce the generation of a gas product from the carbon monoxide-containing hydrotreating step. The gas product is subsequently depleted of hydrogen sulfide and then subjected to water-gas bonding to convert carbon monoxide in the gas to hydrogen. The hydrogen-rich stream resulting from the water-gas displacement is recycled to the hydrotreating stage. The production of this additional hydrogen reduces the hydrogen requirement from external sources, such as replacement hydrogen.

Applicants have now found that by hydrotreating a hydrocarbon fuel of which a portion consists of renewable organic material, it is possible to produce carbon monoxide in the gas in situ, that is, during process operation, thus allowing for simpler and not less production. safe hydrogen solution compared to prior art processes. A highly attractive alternative fuel in the form of a renewable organic material can thus be used without having to resort to highly hazardous solutions such as injecting oxygen from an external source, which additionally requires highly expensive, separate plants to prepare, or highly expensive separate plants. for the provision of additional hydrogen, such as deforming plants for the production of synthesis gas containing carbon monoxide and hydrogen.

Thus, Applicants have performed a process for hydrotreating a hydrocarbon fuel comprising the steps of:

(a) form a feedstock by combining a hydrocarbon fuel containing renewable organic material with a hydrogen current,

(b) conducting the feedstock from step (a) to a hydrotreating stage by contacting said feedstock with at least a fixed bed of hydrotreating catalyst;

(c) passing the effluent from at least one of the fixed decatalyst beds of step (b) to a hot separator and removing from the heat separator a top fraction and a bottom fraction;

(d) passing the top fraction of step (c) to a water-gas displacement stage;

(e) passing the effluent from step (d) to a cold separating and cold separating delay a gas top fraction in the form of a hydrogen rich recycle stream;

(f) passing the gas top fraction of step (e) to a hydrogen sulfide recovery unit in which a solvent is contacted with said gas stream and withdrawing from that recovery unit a reduced hydrogen sulfide gas stream; carbon dioxide, and removing from said recovery unit a solvent containing hydrogen sulfide and carbon dioxide;

(g) return the reduced hydrogen sulfide and carbon dioxide gas stream from step (f) to step (a), return a portion of the reduced hydrogen sulfide and carbon dioxide gas stream from step (f) to step (a) dividing said gas stream into at least two streams and returning at least one of these streams to step (a) and at least one of these streams to step (d).

Preferably, step (g) comprises dividing said stream into two streams and returning one of these streams to step (a) and one of these streams to step (d).

As used herein, the term "renewable organic material" defines vegetable oils, animal fats, tallow oil, tall oil, and derivative material such as fatty acid alkyl esters, particularly fatty acid demethyl esters (FAME) - also known as biodiesel. - or combinations thereof. All of these represent renewable sources. Vegetable oils include rapeseed, soybean, corn, coconut, palm, and cottonseed oils. Animal fats include bacon fat, yellow fat, lard, butter and tallow.

As used herein, the terms "hot separator" and "cold separator" define conventional fractionation units.

By treating a hydrocarbon fuel containing renewable organic material according to the process of the invention, it is possible to convert the in situ produced carbon monoxide and water to CO2 eH2, thus allowing the removal of the generated CO and H2O which may otherwise inhibit the activity of carbon atoms in the hydrotreating unit and limiting their capacity, also due to operational limitations on permitted concentrations of impurities in the cyclic gas stream.

A highly attractive alternative fuel in the form of a diesel, wholly or partly based on renewable sources, can thus be produced without being forced to limit production or to resort to very high bleed rates that are otherwise required to limit contaminant content. as carbon monoxide and which, moreover, has a very low affinity for removal in the hydrogen sulfide recovery unit, such as an amine scrubbing unit.

The hydrogen stream from step (a) is a hydrogen cycle gas stream, a hydrogen replacement gas, or a combination of both.

In one embodiment of the invention, the content of the renewable organic material in the hydrocarbon fuel is at least 5% by volume, preferably at least 20% by volume, more preferably at least 50% by volume. The other component in the hydrocarbon fuel may be a conventional fossil petroleum diesel. At levels above 5% by volume of renewable organic material in hydrocarbon fuel, the carbon monoxide produced reaches a level that makes provision for a water displacement stage expedient. -gas. More preferably, the hydrocarbon fuel contains at least 75 vol% of renewable organic material, particularly at least 75 vol% of biodiesel.

The term "hydrotreating" encompasses a range of process requiring the presence of hydrogen, namely hydrogenation, hydro-cracking, hydro-desaromatization (HDA), hydro-desulfurization (HDS) and hydro-denitrification (HDN). In this invention, the hydrotreating step is preferably a hydrodesulphurization (HDS) step and / or a hydrohydrogenation (HDN) step.

Hydrotreating conditions typically involve operation at temperatures between 200 and 500 ° C, and pressures up to 200 bar. Catalysts used in the hydrotreatment step are preferably employed in conventional manner, such as alumina-supported cobalt and / or nickel and molybdenum mixed sulfides and alumina or silica-supported mixed nickel etungsten sulfides. Other suitable catalysts include those containing ruthenium sulfide and catalysts using novel supports such as silica-alumina, carbons or other materials.

During the hydrotreating stage, the renewable organic material reacts with hydrogen to form a boiling hydrocarbon mixture in the range of diesel, carbon monoxide, carbon dioxide, water, methane and propane. Because the gas phase contains unreacted hydrogen, it is generally recycled to the hydrotreating stage after cleaning. Conventionally, gas cleaning stages include gas purification by amine scrubbing units that remove CO2 and H2S but have a low affinity for CO removal. Thus, carbon monoxide accumulates in the recycle gas circuit unless a significant amount of purge gas is removed and sent for flaring or use in another process unit. If the CO level in the recycle gas becomes too high, the catalyst activity in the hydrotreater is deteriorated and the hydrogen partial pressure is decreased. This also raises the problem that the amount of renewable organic hydrocarbon fuel material needs to be kept as low as 5% by volume.

Since the purge gas contains high amounts of hydrogen, a larger purge means that a higher flow of expensive high purity replacement hydrogen is also required.

By the invention it is possible to operate the hydrotreater with a high content of renewable organic material and at the same time it is possible to obtain a high hydrogen content in the hydrogen rich recycle stream, thus also reducing the need for expensive hydrogen addition through external sources.

The effluent from the hydrotreating stage is passed to a hot separator (fractionation unit), where gaseous products are removed as the light top fraction. Small amounts of hydrogen sulfide may still be present in the stream, which is poisonous to conventional water-gas displacement catalysts, where carbon monoxide reacts reversibly with water to produce hydrogen and carbon dioxide. Thus, it is preferred that the water-gas displacement stage be a sour shift tolerant displacement stage comprising sulfur resistant catalysts such as cobalt-molybdenum catalysts.

Alternatively, the gas top fraction of the hot separator may be scrubbed to remove hydrogen sulfide before passing the same through a conventional water-gas displacement stage. Thus, the invention further comprises passing the gas top fraction of step (c) to a hydrogen sulfide recovery unit wherein a solvent is contacted with said gas stream and withdrawing a reduced sulfide gas stream from said recovery unit. of hydrogen and carbon dioxide, and removing from that recovery unit a solvent containing hydrogen sulfide and carbon dioxide. The stream is then driven to the water-gas displacement stage. As used herein, the solvent is a hydrogen sulfide affinity substance such as amine.

The water-gas displacement stage is selected from the group consisting of minor water-gas displacement, medium water-gas displacement, high water-gas displacement, and combinations thereof. To achieve the advantage of the exothermicity of the CO + H2O = H2 + CO2 displacement reaction, where hydrogen production is favored at low temperatures, but the reaction rate is otherwise slow, high and low temperature displacement can be performed in series. . In a particular embodiment, where hydrogen sulfide has been removed from the gas, the gas-water displacement is high-temperature water-gas displacement conducted at 350-500 ° C with, for example, iron-copper catalysts.

In order to ensure at least stoichiometric amounts of carbon dioxide and water during the displacement stage, steam may be added to the gas stream (top fraction of the step (c) hot separator) or said reduced hydrogen sulfide gas stream and carbon dioxide before conducting said displacement stage. Thus, all carbon monoxide present in the gas is used in the hydrogen production displacement reaction, as the reaction is encouraged to proceed to the hydrogen side.

Subsequent to the water-gas displacement stage, the hydrogen-enriched gas, after resin and mixing with water, is passed to a cold separator (fractionation unit). From this unit, a gaseous top fraction in the form of a hydrogen rich recycle stream is withdrawn along with a bottom fraction in the form of a hydrocarbon liquid stream containing the sulfur free biodiesel and optionally also a sour water stream. containing, for example, deammonium bisulfate caused by removal of some of the ammonium and hydrogen sulfide.

Preferably, the gaseous top fraction of the cold separator is purified in a washing unit, wherein the solvent is an amine. Gaseous effluent stream thus obtained with a reduced content of hydrogen sulfide and carbon dioxide may be partially recycled to the water-gas displacement converter to convert any remaining carbon monoxide to carbon dioxide, or may be completely returned to carbon dioxide. hydrotreating stage. Thus, a portion of the reduced hydrogen sulfide and carbon dioxide stream (step f) is returned to step (d) (water-gas displacement stage) by dividing the gas stream into at least two streams and returning at least one of these streams for the hydrotreating stage and at least one of these streams for the water-gas displacement stage.

The gas stream from the amine scrubbing unit, now representing a purified gas effluent, is used as the hydrogen recycle stream for the hydrotreating reactor and, optionally, as a cooling gas between the catalytic beds of said reactor.

The addition of water to hydrotreating units under conventional hydrotreating conditions is undesirable. The catalytic activity of hydrotreating catalysts is known to be related to the presence of Group VI-B and VIII sulfides such as Co, Mo, Ni, W, in particular alumina-supported Co-Mo, Ni-Mo and Ni-W mixtures. silica, titania, zeolite Y and combinations thereof. If the catalyst is not completely sulfide, its activity decreases dramatically. The presence of water is thus normally avoided in hydrotreating due to the risk that sulfide catalysts will return to their oxidized condition with expected loss of catalytic activity.

Applicants have also found that conventional hydrotreating catalysts are able to establish a quasi-thermodynamic equilibrium for the water-gas displacement reaction at the hydrotreating stage and, by adding water to the hydrotreating stage, the equilibrium of the water-gas displacement reaction is displaced. even more carbon dioxide and hydrogen without affecting the catalyst hydrodesulfurization or hydrodesulfurization capacity.

Thus, in yet another embodiment of the invention, water in an amount from 0.1 to 10% by weight of said hydrocarbon fuel is added to the feedstock of step (a), and / or to one or more fixed hydrotreating catalyst beds. of step (b).

The amount of water added to the hydrotreatment stage depends on the content of renewable organic material in the hydrocarbon fuel. The higher the content of renewable organic material, the greater the amount of water added. For a hydrocarbon fuel containing up to about 20% by volume of renewable organic material, the amount of water added is preferably in the range of 1 to 8% by weight of hydrocarbon fuel, more preferably in the range of 2 to 4% by weight. For a hydrocarbon fuel containing 100% renewable organic material, the amount of water added may be as high as 10% by weight.

Without the addition of water, carbon monoxide formed in situ is at least partially converted to methane under valuable hydrogen consumption. By adding water to the hydrotreating stage, the formation of methane is weakened and the associated consumption of valuable hydrogen is reduced. In addition, a lower CO / CO2 ratio can be obtained and, as a result, there is a lower risk of catalyst poisoning by the presence of carbon monoxide in the hydrotreater. Water may also be added directly to one or more fixed hydrotreating catalyst beds.

In addition, by simply adding water, the amount of renewable organic material, preferably biodiesel, in the hydrocarbon fuel passed to the hydrotreating stage can be significantly increased, for example from about 5% by volume of 10% by volume renewable organic material or 20% by volume or even more, for example 30% by volume or even 100% by volume of biodiesel. Only up to levels of about 5% by volume of renewable material in the feedstock can operate the hydrotreating process without displacement. and / or adding water. The other component in the hydrocarbon fuel may be a fraction of conventional oil, such as light diesel, light cycle oil, coke gas oil, or vacuum oil (VGO).

In yet another embodiment of the invention, water is added to the feedstock and also to one or more fixed hydrotreating catalyst beds.

The single figure shows a simplified diagram of a process according to a specific embodiment of the invention comprising a hydrotreatment stage, associated hydrogen recycle loop, displacement stage and water addition.

Feed stream 1 containing renewable organic material is preheated (not shown) and fed into the hydrotreating reactor 10 after being mixed with hydrogen recycle stream 2 to form stream 3. Stream 3 is still heated (not shown) and sent for at least one catalytic bed 11 of the hydrotreating reactor10. An effluent 4 containing diesel product, hydrogen sulfide, ammonia, carbon dioxide, carbon monoxide, water and other light hydrocarbons is removed from reactor 10 and then resins via the heat exchanger 12 by steam production or preheating of the reactor. food. The cooled effluent is then conducted to hot separator 13 (fractionation unit), where a bottom fraction 5 is removed, while lighter components containing carbon dioxide, carbon dioxide, hydrogen sulfide, ammonia, water, and other light hydrocarbons are recovered. as top fraction 6.

This top fraction is sent for hydrogen recovery via the sulfur tolerant water-gas displacement converter 14, having a fixed sulfur tolerant displacement catalyst 15 bed disposed therein. Steam 7 is added upstream of the sulfur tolerant displacement stage to ensure a complete reaction of carbon monoxide in hydrogen. The effluent 8 from the displacement converter is cooled by steam generation or process change (not shown) and led to a cold separator 16 (fractionation unit). From the Mo separator, a hydrogen-rich top fraction is removed and passed to an amine scrubbing unit 18. Purified gas effluent 2 of amine scrubbing unit 18 is partially used as the hydrogen-cyclic stream to the hydrotreating reactor. 10 and optionally as a cooling gas between the catalytic beds 11. A resulting portion 2 is recycled as a stream 19 to the water-gas displacement converter 14 to ensure complete removal of carbon monoxide. Water is added as stream 21 to hydrocarbon feedstock 3 and as stream 22, 23 to one or more fixed beds of hydrotreating catalyst 11. A bottom fraction 9 is then removed from cold separator 16 containing diesel product for other use as a fuel for clean renewable transportation.

Example 1

Feed Load 1 (Table 1) is a mixture of 20% by volume of rapeseed oil and 80% by volume of vacuum gas oil. Pure vacuum gas oil is denoted feed charge 2. Feed charge A was hydrotreated in a direct production pilot plant using 105 ml of a hydrotreating catalyst (Ni and Moimpregnated onto an alumina support). The flow rate of the Afoi feed charge 105 ml / h, corresponding to an LHSV of Ih "1. 100% hydrogen (semi-cycle) at a pressure of 80 bar was co-fed with a deliquid current at a rate of 735 Nl / h, corresponding to an H2 / oil ratio of 700Nl / 1. Reactor temperature was maintained at 350 ° C. Product samples were taken from liquid effluent Reactor outlet gas was analyzed by gas chromatography.

Table 1

<table> table see original document page 14 </column> </row> <table> <table> table see original document page 15 </column> </row> <table>

Based on oil and hydrogen flows, liquid product analyzes and outlet gas composition, Table 2 was constructed, showing yields (expressed as (g product / g liquid feed) x 100%), and gas composition based on volume.

Table 2

<table> table see original document page 15 </column> </row> <table> H2 Consumption (Nl / 1)

<table> table see original document page 16 </column> </row> <table>

As can be seen from Table 2, the main differences when processing feedload B compared to processing a pure hydrocarbon feedload that does not contain or contain only trace amounts of oxygen is the emergence of CO, CO2, and H2O, as well as increased yields of CH4e C3H8 in the gas phase product. The increase in C3H8 yield is in line with experimental uncertainty, consistent with the completed conversion of triglycerides to rapeseed oil. The conversion of HDS measured in this as in the following examples was approximately 96-97%.

This example shows that considerable amounts of CO, CO2 and H2O are formed when processing an oxygen-containing feedstock such as vegetable oils and / or animal fats. In addition, the high CH4 yield indicates that the formed CO is partially removed in CH4, thus consuming valuable hydrogen.

The equilibrium constant Kp for the gas-water displacement reaction can be, for an ideal gas mixture, written as:

KP = (yco2yH2) / (ycoyH2o),

where yx denotes the volumetric gas phase concentration of compound X (X = CO2, H2, CO2, H2O). At 350 ° C, the theoretical value of Kp is 20.7. From experimental data, a value of 17.6 is calculated, very close to the theoretical equilibrium value. This example thus also shows that a hydrotreating catalyst is effective in establishing thermodynamic equilibrium for the water-gas displacement reaction.

Example 2

Example 1 feed load 1 is processed using the same catalyst and conditions (temperature = 350 ° C, gauge pressure = 80 bar. LHSV = 1 h "1, H2 / oil = 700 Nl / 1) as in example 1 in However, H 2 O was added to the feed before entering the reactor Two experiments with 2.1 gH20 / h and 4.2 gH20 / h respectively were conducted These water flows correspond to 2.0 vol% and 4.0 vol% by volume. feed charge flow A. The results are shown in Table 3, the yields calculated based on the new feed charge flow (excluding H2O).

Table 3

<table> table see original document page 17 </column> </row> <table> <table> table see original document page 18 </column> </row> <table>

In these tests, the sulfur and nitrogen conversion was similar to that observed in Example 1. The total molar yield of CO5 CO2 and CH4 in example 1 and example 2 is within the 5% constant for all A-load, with or without injection, experiments. of water. This shows that CO and CO2 formed from decarboxylation of rape seed oil will only react on CH4, and not on any other compounds. When H2O is injected, the yield of CH4 decreases, meaning that less CO and CO2 is transformed into CH4. The formation of CH4 and the associated hydrogen consumption are not attractive and this example shows that it can be suppressed by injecting H2O.

In addition, H2O injection also shifts the water-gas displacement balance to CO2 and H2. As can be seen from Table 3, the observed Kp values for the water-gas displacement reaction (16.3 and 16.9) are close to the thermodynamic equilibrium value (20.7) and the value observed in Example 1 (17 , 6), meaning that the catalyst is still effective to establish equilibrium in the water-gas displacement reaction, also in the case where H2O is added to the feed.

Consequently, the molar ratio C0 / CO2 decreases from 1.5 when not added H2O (Example 1) to 0.77 (2.1 g H2 O / h added) and 0.49 (4.2 g H2 O / h added) . In addition, the absolute level of CO in the outlet gas decreases from 0,48% by volume to 0,37% by volume (2,1 g H20 / h added) and 0,29% by volume (4,2 g H20 / h h added). When effluent gas is recycled to the reactor inlet, CO and CO2 will accumulate in the recycle loop if steps are not taken to prevent this. As CO eCO2 can inhibit catalyst activity, it is advantageous to remove CO2 from the treatment dog, for example by an amine wash step. Amine leaching does not remove CO, but this example demonstrates that by injecting H2O, lower CO levels in the treatment gas can be achieved without additional hydrogen consumption.

When larger amounts of H2O are present in the reactor, water-gas displacement area is shifted to CO2 and H2. Additionally, CO methanation in CH4 is suppressed. These effects lead to lower hydrocarbon consumption (141 and 140 Nl / 1) when H2O is injected compared to Example 1, where feed load A is processed in the absence of H2O (H2 148 Nl / 1 consumption). The present examples thus demonstrate that by injecting H2O, global hydrogen consumption can be decreased.

Claims (10)

Process for hydrotreating a hydrocarbon fuel, comprising the steps of: (a) forming a feedstock by combining a hydrocarbon fuel containing renewable organic material with a hydrogen stream, (b) driving the feedstock of the stage. (a) at a hydrotreating stage by contacting said feedstock with at least a fixed bed of hydrotreating catalyst, (c) passing the effluent from at least one of the fixed decatalyst beds of step (b) to a hot separator. and removing from the heat separator a top fraction and a bottom fraction, (d) passing the top fraction of step (c) to a water-gas displacement stage, (e) passing the effluent from step (d) to a separate one. cold and cold separating delay a gas top fraction in the form of a hydrogen-rich recycle step stream (f) pass the gas top fraction of step (e) to a recovery unit hydrogen sulfide wherein a solvent is contacted with said gas stream and withdrawing from said recovery unit a reduced hydrogen sulfide and carbon dioxide gas stream and withdrawing from said recovery unit a solvent containing hydrogen sulfide and dioxide (g) return the reduced hydrogen sulfide and carbon dioxide gas stream from step (f) to step (a), or return a portion of the reduced hydrogen sulfide and carbon dioxide gas stream from step ( f) to step (a) by dividing said gaseous stream into at least two streams and returning at least one of these streams to step (a) and at least one of those streams to step (d). A process according to claim 1, characterized in that the hydrogen stream of step (a) is a hydrogen cycle gas stream, a hydrogen replacement gas or a combination thereof. Process according to Claim 1 or 2, characterized in that the content of the renewable organic material in the hydrocarbon fuel is at least 5% by volume. A process according to any one of claims 1 to 3, characterized in that the hydrotreating step is a hydrodesulfurization (HDS) and / or hydrodesionitrogen (HDN) step. Process according to any one of claims 1 to 4, characterized in that the water-gas displacement stage is a sulfur tolerant displacement stage. Process according to Claim 1, characterized in that it further comprises passing the gas top fraction of step (c) to a hydrogen sulfide recovery unit wherein a solvent is contacted with said gas stream and withdrawing from said recovery unit. , a gas stream with reduced hydrogen sulfide and carbon dioxide content, and removing from that recovery unit a solvent containing hydrogen sulfide and carbon dioxide. Process according to any one of claims 1 to 6, characterized in that the water-gas displacement stage is selected from the group consisting of minor water-gas displacement, medium water-gas displacement, high water-gas displacement, ecombinations of the same. Process according to any one of claims 1 to 7, characterized in that before conducting the water-gas displacement stage, steam is added to said top fraction of step (c) or said low-gas content. hydrogen sulfide and carbon dioxide. 9 Process according to any one of claims 1 to 8, characterized in that water in an amount of 0.1 to 10% by weight of said hydrocarbon fuel is added to the feedstock of step (a), and / or o one or more fixed hydrotreating catalyst beds of step (b).